1. Field of the Invention
This invention relates generally to compositional analysis techniques of materials and devices, and more specifically to a method and apparatus for analyzing the chemical composition and physical distribution of molecular substances in solid materials, such as semiconductors and similar devices.
2. Description of the Prior Art
It is well-known and generally accepted that single crystal materials, such as silicon, indium, copper, selenium, and others make the most effective and efficient semiconductor materials for use as solar cells and other electronic devices. It is also known and generally understood that the time and meticulous controls necessary for growing such single crystalline materials make them uneconomical for large-scale or mass produced uses, at least under present technological limitations in this field. As a result, more attention has been directed recently toward less perfect, but more easily and less expensively produced materials, such as polycrystalline and amorphous silicon and other similar materials for use as solar cells and other semiconductor devices.
Polycrystalline materials, while easier and less expensive to produce than single crystal materials, generally do not make as good or efficient semiconductor devices as single crystal materials. The crystalline grain boundaries or other defects in polycrystalline materials adversely affect the performance of these materials as semiconductors, and particuarly as effective photovoltaic converters or solar cells.
Both the understanding and control of the behavior of grain boundaries, particularly in polycrystalline silicon (Si), have advanced substantially in recent years. As a result, the adverse effects of the grain boundaries on the performance of polycrystalline Si semiconductors is now attributed to essentially four mechanisms. First, these intercrystalline boundaries can impede the flow of majority charge carriers, increasing the series resistance of the device. Second, dangling or unfilled bonds at the grain boundaries can provide electrical shunts across the polycrystalline layer, thereby reducing both the fill-factor and the open-circuit voltage. Third, impurities in the grain boundaries can act as recombination centers across the forbidden band gap for the minority carriers, thus decreasing the electric field across the junction. Fourth, these grain boundaries can serve as enhanced diffusion paths for impurity species into the semiconductor device, which can react and degrade the chemical structure of the material, thus limiting and decreasing the operational lifetime of the solar cell.
Efforts are now being made to develop methods and techniques for minimizing or eliminating these adverse effects of grain boundaries in crystalline semiconductor and solar cell materials. For example, techniques have been developed to segregate oxygen and other impurities to the grain boundaries by heat treatments or high-temperature processing and then passivating these grain boundaries by incorporation of hydrogen therein to form silicon hydroxide (SiOH) molecules, which have completely satisfied bonds, thus reducing the deleterious effects of dangling bonds at the grain boundaries.
This technique and others show promise of improving the performance of polycrystalline solar cells and semiconductor devices. However, prior to this invention, there was no effective technique available for obtaining data and analyzing the precise chemical and compositional makeup of the grain boundaries including the impurity distribution in the grain boundaries. Thus, it has been very difficult, and largely an educated guessing process, to determine the chemical substances present in the grain boundaries and the effectiveness of various correctional techniques.
Attempts have been made to fracture polycrystalline materials along grain boundaries to expose these interfaces to direct chemical analysis. However, this technique has several serious problems. First, it is very difficult to fracture the material precisely where analysis is desired. For example, after gaining experience, about one out of 60 fractures of a polycrystalline silicon sample is successful at exposing the desired region. Even then, a successful fracture exposes only part of the grain boundary. The other part is lost, usually to the bottom of the analysis chamber. Thus, there is a high probability that the real information of interest to the problem may be lost. Further, multiple or subsequent processing (e.g., diffusion or heat treatment) cannot be accomplished on a given sample segment since the fracture procedure is violently destructive.
Other techniques have included the use of conventional surface analysis methods, such as Auger electron spectroscopy (AES), secondary ion mass spectroscopy (SIMS), or X-ray photoelectron spectroscopy (XPS) in conjunction with sputter etching. These conventional techniques are focused on a point on the material's surface and are adapted to detect the chemical makeup of the material on that point. Therefore, when combined with sputter etching of the surface to provide depth, the point of analysis becomes a line analysis. Such techniques provide compositional information about the elements and molecular species along a line perpendicular to the surface. However, if the intent is to observe and resolve the composition of a plane (e.g., the map of impurities that may have segregated to some internal grain boundary or surface), these conventional profiling techniques cannot be utilized. They would give information only on a small region or point on the internal surface or grain boundary, provided the analyst is fortunate enough to take the data while passing through the internal surface or grain boundary during analysis. Further, while grain boundaries are very thin and appear as surfaces, they are actually three-dimensional regions where impurities can concentrate, so it is really not possible to observe the actual chemical and compositional make up of a grain boundary region from the point or even line data obtainable with conventional AES, SIMS, or XPS analysis techniques, even when they are performed in conjunction with sputter etching.